I have both an iPhone and an android based Nexus 7.
With both there are challenges:
The iPhone requires a Mac to program. I know some will scream that this isn't so because you can program through a Windows machine and port it but you will lose all of the support asosociated with the Objective C programming environment and I have read that it isn't stable. So trust me you will need a mac. I purchased a Mac Mini for this purpose. Then there is the problem of having to pay Apple for the priviledge of putting an app on your phone. This is where I have the biggest problem. This isn't just for the itunes store but even just to put a simple 'Hello World' app on your device you have to purchase a license. This is a major problem for me. Next you have the actual interface. There are cables you can buy that allow you to hard wire interface with an arduino and this is a good option. In order to use bluetooth, you have to purchase a bluetooth adapter chip through apple for the electronics side. You have to have this chip for every device you build.
The only real challenge with the Android system is learning a new language if you are unfamiliar with Java. From the time I installed the IDE to the time I had my first app on my Tablet was about an hour and that was mostly spent going over the code to understand what I was doing. You could put one of the sample apps on your device in about 2 minutes. You have full access to bluetooth without any special decoding chip. I don't know of a cable interface but I don't see the point when you can use bluetooth and keep it clean. I got a bluetooth module from Sparkfun for about $20 but have made no effort as of yet to use it.
Hope that helps